BRAOHIOPODA. 791 
Rhynchospira, Hall. 1859. 
(Plate 36, figs. 8-12.) 
Shells elongate, retziiform ; hinge-line short and curved. Umbo 
of the pedicle-valve incurved, usually concealing the deltarium ; 
apex truncated by a circular foramen. Cardinal slopes gradual, 
scarcely excavated, not forminga false area. Deltarium triangular 
and flat or incurved ; its lateral margins are sharply defined, and 
its surface traversed by a longitudinal median ridge, which is the 
line of solid coalescence of the constituent plates. The umbonal 
cavity does not contain the split deltidial tube which is present 
in the genera Retzia, Husrepra, etc. The teeth are small and 
well defined, and are not supported by dental plates. Muscular 
impressions very obscure. 
In the brachial valve the hinge-plate has the general form of 
that in Tremarospira, but is much less elevated. Its posterior 
extension is slight, extending but a short distance beyond the 
hinge ; it consists of two parts, a lower, which is closely appressed 
against the umbo, and deeply divided by a median cleft; and an 
upper, which is larger, conspicuously elevated and divided medially 
only at its margin, though the groove extends forward to the 
middle of the plate. The anterior portion is deeply concave and 
produced into two flat lobes which form the crural bases. The 
entire plate rests on stout supports which diverge at the bottom, 
leaving a triangular cavity beneath, in which there is a short, 
sometimes obscure median septum. The spirals make from six 
to nine volutions, the primary lamelle being nar- 
row and not greatly incurved. The jugum is 
situated behind the middle of these lamelle and 
is simple in its structure; its lateral branches 
narrow just above their origin, with a gentle poste- 
rior inclination, then broaden and meet at a little ee Saracen 
more than one-half the distance across the base of spira formosa, Hall. 
the coils, forming a broad, short, roof-shaped process, which is 
directed posteriorly and terminates in an oblique edge. 
The external surface is radially plicate, the plications being 
simple. In young shells there is a median sinus on both valves, 
but as growth advances, that of the brachial valve develops into 
alowfold. Both fold and sinus bear a number of small, intercalary 
43 
